Blocking diodes in PV arrays

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weber
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Blocking diodes in PV arrays

Post by weber » Mon, 22 Jun 2015, 15:33

I have started this thread to continue a discussion that began as an off-topic digression in the Tesla Powerwall thread. If you haven't already, you should begin reading here:
viewtopic.php?title=tesla-powerwall&p=5 ... 529#p57391

It's about whether blocking diodes are required when solar panels are first connected in series to form strings, and then multiple strings are connected in parallel. All panels are of the same type and all strings have the same number of panels.
Last edited by weber on Mon, 22 Jun 2015, 07:16, edited 1 time in total.
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Blocking diodes in PV arrays

Post by weber » Mon, 22 Jun 2015, 17:57

lopezjm2001 wrote: That was a good detailed reply Weber. I expect that using two strings in parallel, one string facing east and one facing west will become more common to shift the solar power to morning and afternoon peak periods using the same MPPT input for both strings. Thus reducing power going to grid at mid day at the low infeed tariff. This allows the designer to use a cheaper inverter which only has one MPPT input instead of two. This is one example where you expect one string to be shaded (or just exposed to ambient light) while the other is in full sun on most days. Totally legal along as they are the same number of modules in each string and all modules are the same. No need for a blocking diode.

That's correct. AS 5033 only requires that "PV modules that are electrically in the same string shall be all in the same orientation +-5° (azimuth and tilt angle)". (from section 2.1.6)

I should point out where I stretched the truth in the interests of round figures, for an easy-to-follow example in my previous post. It would be unusual for panels to rise 40 degrees above ambient in full sun unless they had been mounted too close to the roof so as not to allow sufficient air circulation under them, or they were laid nearly horizontal and there was no wind. A more typical temperature rise would be 25°C to 30°C. But it would also be unusual for a shaded panel to have only 1% sun as there is always some diffuse light coming from all parts of the sky in the daytime. These two liberties would however tend to cancel out and give much the same result -- very little reverse current due to shading, even with an open circuit array at high voltage.

Compliance with AS 5033 "Installation and safety requirements for photovoltaic (PV) arrays" is a legal requirement because it is required by AS 3000 "Electrical Installations (Wiring rules)" and compliance with AS 3000 is required by the various state Electricity Acts.

AS 5033 section 4.3.10 says "Blocking diodes may be used but they are not a replacement for overcurrent protection." And it requires that, if they are used, they must be rated for at least 1.4 times the string's (and hence panel's) rated short circuit current (at STC), and 2 times the string's worst-case open circuit voltage (considering the lowest expected temperature).
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Blocking diodes in PV arrays

Post by weber » Mon, 22 Jun 2015, 19:44

Thanks lopezjm2001 and T1 Terry.

lopezjm2001, I note that you would get slightly more total energy if you had west and east strings on separate MPPTs, and even this would be about 15% less than if you had both strings facing north (assuming 25 degree roof slopes at latitudes around those of Brisbane or Perth). But as you say, now that panels are cheap and feed-in tariffs almost non-existent, there may be good economic reasons for east and west with one MPPT.

Terry, I didn't address your question:

"Am I correct in assuming the shaded solar panels will be the end of the reverse current flow as the output voltage below this point would be common between the strings?"

And so you continued in a similar vein with:

"... well at least the last one in the string as this is the panel that would suffer reverse current flow".

No. I'm afraid that's not correct. You need to think of the electrons as being like the carriages of a train, or like the molecules of a liquid under pressure, in the sense that they can't pile up on each other, and they can't open up gaps between themselves (except inside capacitors, which we don't have here). If they try to pile up, the electrostatic repulsion of their alike (negative) charges pushes them apart. If they try to open up a gap, the attraction from the oppositely (positively) charged atoms they leave behind pulls them back again.

A current is the rate at which electrons pass a given point. One amp is 6,241,509,750,000,000,000 electrons per second (give or take 10,000,000,000 Image ), also called one coulomb per second.

So when a number of things are connected in series, the current must be the same at any point. A blockage anywhere stops traffic flow everywhere in that series circuit, on both sides of the blockage.
T1 Terry wrote:Thankyou Weber, I have a watt meter that I can put in line with string B and as long as I also monitor panel temp ... I can do a hands on test.
Why would you use a watt meter? Don't you want an ammeter (or a multi-meter on its 10 amp (or higher) range)? Either the kind you can clamp around a wire (although these are rarely accurate to better than +- 0.5 Adc) or the kind with two probes where you have to break the circuit to insert it.

A watt meter must measure both volts and amps simultaneously and needs at least 3 connections. Unless you're talking about the kind of crude "watt meter" designed for 240 Vac, which assumes 240 volts and cannot measure DC current or power.
I have tested and found a similar effect [f]or the short term reverse current flow with single panels in parallel, but multiple panels in series complete with bypass diodes have always dropped current greater than the expected loss of 1 panel,
The problem here is that the expected power loss is not just the loss of the power from the one shaded panel. It is the loss of the current from the whole string containing the shaded panel and therefore the loss of the power from _all_ panels in that string. Unless ...
so now I will try to determine why if it is not the result of reverse current flow into the shaded panel. I conduct the tests as a direct connection (with circuit breaker) to the relative voltage battery pack to eliminate any effects caused by the MPPT controller
Unless you have an MPPT, _and_ you have at least one more panel in each string than you would need to charge the battery without an MPPT, then the voltage required to extract any power from the string with the shaded panel will be below the battery voltage, and therefore unable to deliver any power to the battery.

To extract power from the string with the shaded panel, the array voltage has to be lowered (by the MPPT or the battery) to the point where the bypass diode(s) around the shaded panel begin to conduct and allow the 3 unshaded panels to produce current by bypassing the shaded panel. That voltage will be around 3 x Vmp = 51 V, minus the diode drop of about 0.6 V.

Since MPPTs only do "buck" conversion, not "boost", not even an MPPT can help you with a shaded panel, if you only have 4 x 36 cell panels in each string for charging a nominally 48 volt battery.

If you don't have excess voltage available from your array (which in normal operation an MPPT will convert to extra battery current) then a single shaded panel (possibly even a single shaded cell in hot weather) will result in losing the entire output of the string it is in.

That's one reason why both myself and solamahn run the MPPT of the PIP-4048MS with 3 x 72-cell panels in each string (nominally 72 V) even though the battery is nominally 48 V.
Last edited by weber on Mon, 22 Jun 2015, 09:49, edited 1 time in total.
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Blocking diodes in PV arrays

Post by lopezjm2001 » Tue, 23 Jun 2015, 02:27

weber wrote: lopezjm2001, I note that you would get slightly more total energy if you had west and east strings on separate MPPTs, and even this would be about 15% less than if you had both strings facing north (assuming 25 degree roof slopes at latitudes around those of Brisbane or Perth). But as you say, now that panels are cheap and feed-in tariffs almost non-existent, there may be good economic reasons for east and west with one MPPT.
East/west strings are way more efficient on a flat roof using tilt frames. Almost all commercial roofs and the odd residential house roof is flat and ideal. As no spacing is required between string rows to avoid shading from the adjacent row in winter. I know of one house around the block that has a flat roof with 5 rows of seven modules each all facing north at about 30deg tilt. Ideal design 5 years ago when the solar bonus scheme was available with the 60c/kwh infeed tariff. After the solar bonus scheme finishes in 2017 the owner can consider near doubling the size of the array using a east/west facing configuration.
Last edited by lopezjm2001 on Mon, 22 Jun 2015, 16:35, edited 1 time in total.

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